The present invention is directed to a method for estimating a vehicle speed of a four-wheel motor vehicle for determining slip rates of front and rear wheels in an anti-lock brake system, and more particularly to a method wherein a vehicle speed is estimated independently on front wheel side and rear wheel side on the basis of front wheel speeds, respectively and rear wheel speeds.
In order to effect an anti-lock control for brakes mounted on front wheels and rear wheels, it is a conventional practice to estimate a vehicle speed required for an anti-lock control of the front wheel brakes on the basis of front wheel speeds and estimate a vehicle speed required for an anti-lock control of the rear wheel brakes on the basis of rear wheel speeds. A slip rate is determined by comparison of reference wheel speeds determined by these estimated vehicle speeds with the wheel speeds for the anti-lock control. Such estimating methods are known, for example, from Japanese Patent Application Laid-open Nos. 41237/82 and 60948/82.
When a vehicle equipped with an anti-lock control brake device is braked during travel on a straight road, the highest wheel speed of the four wheels is a value nearest to the vehicle speed whether the brake device is or is not under an anti-lock control. Thereupon, if a vehicle speed on the front wheels is estimated on the basis of a high select value of the front wheel speeds and a vehicle speed on the rear wheels is estimated on the basis of a high select value of the rear wheel speeds, such estimated vehicle speeds can approximate to the actual vehicle speed when the vehicle is braked during travel on a straight road. When the vehicle is braked during turning thereof, however, a large difference is produced between the highest one of the four wheel speeds and any other wheel speed due to inner and outer wheel speed differences of the left and right wheels. For this reason, if a vehicle speed is estimated on the basis of a high select value as described above, anti-lock control may be performed unnecessarily due to a determination of excessive slip having been produced since a reference wheel speed that indicates a slip rate is determined by the estimated vehicle speed.